The present invention relates to a quantum cascade laser and, more particularly, to a quantum cascade laser structure that is capable of self-mode-locking behavior in the mid-infrared wavelength range.
Over the past few decades, there has been an extensive research effort focused on the development of xe2x80x9cultrafastxe2x80x9d laser sources, that is, sources capable of generating optical pulses with durations ranging from a few picoseconds down into the femtosecond range. Some of these ultrashort pulses have been generated in a variety of gas and solid-state laser media. These sources have allowed for a dramatic improvement in the temporal resolution of a myriad of measurements in physics, chemistry and biology. The need for increased bandwidth in optical communications is another factor influencing the design and development of ultrafast laser sources. Ultrafast semiconductor lasers are particularly important for telecommunications applications, given their compact size, high efficiency, low cost and unmatched pulse repetition rates (up to a few hundred of GHz).
The most commonly used approach to the generation of ultrashort laser pulses is the technique of mode-locking. In general terms, mode-locking results from a periodic modulation of the laser gain with the fundamental period equal to the cavity roundtrip time. Under these conditions, maximum gain is experienced by a laser beam consisting of a train of pulses, separated by the roundtrip time, and properly synchronized with the modulation. An optical waveform with these characteristics is established through the coherent addition of several longitudinal modes of the laser cavity, when the modes are phase-locked to one another. The characteristics of such a device are often described in the frequency domain. In this description, when the laser is modulated at the cavity roundtrip frequency (i.e., the frequency separation between adjacent modes), several modes are driven above threshold by the modulation sidebands of their neighbors, which automatically establishes the phase-locking required for pulsed laser emission.
In general, the modulation responsible for mode-locking may be produced by an external source (defined as xe2x80x9cactivexe2x80x9d mode-locking), or by the laser pulses themselves through some intracavity optical nonlinearity (defined as xe2x80x9cpassivexe2x80x9d, or xe2x80x9cselfxe2x80x9d mode-locking). Typically, the shortest pulse durations and the largest repetition rates can be obtained with self-mode-locking (SML) and several techniques of SML have been demonstrated over the past few years. In each case, a nonlinear mechanism is required that reduces the losses with an increasing optical power. One exemplary arrangement is an intracavity saturable absorber; that is, an absorber whose opacity at the laser wavelength decreases with increasing intensity. Alternatively, a nonlinear mirror or a nonlinear coupled-cavity (with larger reflectivity at higher power levels) may be used. Another effective mechanism, discovered in association with Ti:sapphire lasers, is self-focusing or Kerr-lensing, which requires an intracavity medium with a positive nonlinear refractive index. That is, a refractive index that increases with increasing intensity. In such a medium with a positive nonlinear refractive index, the center part of the beam transverse profile (where the intensity is higher) experiences a larger index, and is therefore slowed down in its propagation relative to the edges. Thus, the nonlinear medium acts as a positive lens narrowing the beam diameter, to an extent proportional to the optical power; this effect can then be converted into a saturable loss mechanism simply by using an intracavity slit or aperture.
Regardless of the nature of the nonlinear mechanism, it is essential that the loss recovers from saturation on an xe2x80x9cultrafastxe2x80x9d time scale, in particular, on a time scale that is much faster than the cavity roundtrip time. In other words, after the passing of each pulse, the losses must quickly return to their steady-state (relatively high) value before the arrival of the next pulse, so as to prevent light emissions between consecutive pulses. The relaxation lifetime of the SML nonlinearity also limits the resulting optical pulse widths. For these and other reasons, all prior art demonstrations of self-mode-locking having relied upon an ultrafast nonlinearity, either provided by an external medium added inside the cavity, or by a nonresonant transition in the laser host medium.
In principle, an xe2x80x9cintrinsicxe2x80x9d nonlinear refractive index is present in any laser medium, provided by the lasing transition itself and related to the gain coefficient through a Kramers-Kronig transformation. This is a resonant nonlinearity, and therefore inherently large, so that one may expect that, combined with the appropriate cavity configuration (e.g., a coupled-cavity system, or an intracavity aperture), a laser medium with a sufficiently large intrinsic nonlinear refractive index could be used to provide a self-mode-locking laser. However, since such nonlinearity involves a real population transfer across the lasing transition, its dynamic response is limited by the lifetime of the upper laser state. In mode-locked lasers developed to date, this lifetime is much slower than (or comparable to) the cavity roundtrip time, and therefore fails to satisfy the above-mentioned requirements for SML.
In general, therefore, mode-locked lasers of the prior art have been limited to an xe2x80x9cextrinsicxe2x80x9d structure, requiring the use of an externally added non-linearity, or loss xe2x80x9cdiscriminatorxe2x80x9d to provide the mode locking capability. The use of one or more external components increases both the cost and complexity of the mode-locked laser system.
Thus, a need remains in the art for an intrinsic self-starting/self-sustaining mode-locked semiconductor laser arrangement, i.e., a laser system where the laser transition itself provides the nonlinear component of the mode-locking mechanism.
The need remaining in the prior art is addressed by the present invention, which relates to a self-mode locking laser and, more particularly, to a quantum cascade laser structure that is capable of self-mode-locking behavior in the mid-infrared wavelength range.
In accordance with the present invention, quantum cascade lasers characterized by intersubband transitions having large index nonlinearities (due to their extremely large dipole moments) are used to generate picoseconds pulses of mid-infrared light. In particular, Kerr-lens mode-locking of a QC laser is provided by the index nonlinearity of the intersubband lasing transition.
The intracavity aperture required to convert the QC self-focusing mechanism into a loss modulation is provided by a QC laser waveguide that is characterized by: (1) an optically highly lossy (i.e., absorbing) layer (such as a metal, for example), separated from the semiconductor material by a relatively thin dielectric layer such that it xe2x80x9cseesxe2x80x9d (i.e., optically interacts with) the optical wave formed in the active region; and (2) a relatively long laser waveguide (such that the propagation losses dominate over other losses, such as mirror losses). In one embodiment, the highly lossy layer used for mode coupling can comprise a metal and be disposed to also form one of the electrical contacts for the laser device.